Inmarsat Data Explainer
The Inmarsat satellite handshake data is the only evidence placing MH370 in the southern Indian Ocean. Seven hourly "pings" between the aircraft and the Inmarsat-3 F1 satellite form the basis of every official search. This page explains what the data actually shows, how it's been interpreted, and where the analyses diverge.
The Only Evidence for the Southern Indian Ocean
After MH370 disappeared from military radar near Penang at 18:22 UTC on March 7, 2014, the only trace of the aircraft came from automated "handshake" signals between its satellite data unit (SDU) and the Inmarsat-3 F1 geostationary satellite. These handshakes (essentially electronic check-ins) occurred roughly every hour for nearly seven hours after the last radar contact.
The handshakes contain two key measurements: Burst Timing Offset (BTO), which indicates the aircraft's distance from the satellite, and Burst Frequency Offset (BFO), which indicates whether it was moving north or south. Together, these two data points form the entire basis for concluding that MH370 flew south into the Indian Ocean.
Mixed: the raw telemetry is genuine and has been independently verified. The interpretation methodology (how distance rings and Doppler data translate to a specific crash location) is where analyses diverge significantly.
- Malaysian Safety Investigation Report (2018), Section 1.9
- Inmarsat satellite handshake log (released May 2014) — Released to media; no official persistent URL
How Satellite Handshakes Work
Inmarsat's system performs periodic "log-on interrogation" requests. The aircraft's SDU responds automatically; no crew action required. Each exchange produces two measurable values.
BTO: Burst Timing Offset
Measures the round-trip time for the signal to travel from the satellite to the aircraft and back. Since the speed of light is constant, this time directly maps to distance. Each BTO value defines a ring of possible positions; the aircraft was somewhere on a circle at that specific distance from the satellite.
Think of it like a compass: you know how far away something is, but not in which direction.
BFO: Burst Frequency Offset
Measures the Doppler shift of the signal frequency. When the aircraft moves toward the satellite, the frequency shifts up; when moving away, it shifts down. The magnitude and sign of this shift indicate the aircraft's velocity component toward or away from the satellite, which effectively reveals whether it was flying north or south.
This is what eliminated the northern corridor and pointed to the southern Indian Ocean.
Why only these two data points? The Inmarsat system wasn't designed for aircraft tracking. It's a communications relay satellite; the BTO and BFO are engineering byproducts of the handshake protocol, not navigational data. Inmarsat engineers discovered they could extract location information from these values only after MH370 disappeared.
The Official Interpretation
The Australian Transport Safety Bureau (ATSB) and Defence Science and Technology Group (DSTG) developed the primary analytical framework used to guide the underwater search.
Bayesian Analysis Method
The DSTG paper (Bayesian Methods in the Search for MH370, 2016) describes their approach: combine BTO/BFO data with aircraft performance models (fuel consumption, speed, altitude constraints for a Boeing 777) using Bayesian probability methods. The result is a probability distribution along the 7th arc, with the highest probability hotspot at approximately 33°S–35°S.
Result: Southern Corridor, 7th Arc
The BFO data eliminated the northern corridor (the aircraft was flying south, not north). The BTO rings, combined with aircraft performance limits, narrowed the final position to a segment of the 7th arc in the southern Indian Ocean. Over 240,000 sq km of ocean floor has been searched along this arc. No main wreckage has been found.
Key Assumption: Normal SDU Reboot
At 18:25 UTC, the SDU performed what appears to be a power-up log-on, as if it had been switched off and back on. The official interpretation treats this as a normal reboot (possibly triggered by fuel exhaustion of the left engine, causing a brief power interruption). This assumption is critical: if the SDU reboot was abnormal or externally triggered, the entire interpretation of the final two handshakes changes.
Competing Analyses
ATSB, the Independent Group, Godfrey's WSPR method, and Forbes each reach different conclusions from the same satellite handshakes. Their methodologies and confidence levels diverge sharply.
Method: Bayesian analysis incorporating aircraft performance models (fuel, speed, altitude constraints), BTO/BFO satellite data, and drift studies. Probabilistic hotspot analysis along the 7th arc.
Assessment: High: peer-reviewed methodology, independently verified by multiple academic groups. However, the 120,000+ sq km of negative search results raise questions about input assumptions.
Method: Combines forward satellite data analysis with reverse-drift modelling from confirmed debris recovery locations. Cross-references BTO/BFO with actual oceanographic data to refine the impact point.
Assessment: High: uses the same satellite data as ATSB but adds independent debris-based constraints. Prediction falls within the 2026 search zone.
Method: Weak Signal Propagation Reporter (WSPR) analysis using amateur radio signal anomalies to triangulate aircraft positions along the flight path. Uses HF radio propagation disturbances caused by the aircraft.
Assessment: Disputed: methodology is novel and contested. Multiple RF propagation experts have published critiques arguing WSPR signals lack the resolution for aircraft tracking. Godfrey maintains the approach is valid.
Method: Argues the Inmarsat data may have been fabricated or manipulated to misdirect the search. Points to anomalies in the data and the lack of wreckage as circumstantial support. Position is that the satellite footage shows a different fate for the aircraft.
Assessment: Speculative: requires assuming the data isn't genuine. The Inmarsat telemetry has been independently verified by multiple groups. However, the interpretation of what the data means is where analyses diverge.
The "10 Suspicious Rows"
Forbes has highlighted specific data rows in the Inmarsat log that he argues show signs of fabrication or manipulation. This is one of his more specific technical claims, and it has received detailed rebuttals from the independent analysis community.
Forbes' Claim
Ten specific rows in the published Inmarsat data contain values that are anomalous or inconsistent with the surrounding data.
These anomalies suggest the data may have been inserted or modified after the fact to support the southern Indian Ocean narrative.
If these rows are removed, the remaining data no longer clearly supports a southern flight path.
Independent Rebuttal
The Inmarsat data has been independently verified by Inmarsat engineers, the UK AAIB, DSTG, and multiple independent researchers (Iannello, Ulich, Holland).
The "anomalous" values have technical explanations: SDU warm-up effects, satellite oscillator drift, and normal variation in the handshake protocol. These are documented in the DSTG paper.
Fabricating the data would require simultaneously spoofing BTO, BFO, and log-on/log-off sequences in a way that is internally consistent, across Inmarsat's own infrastructure, which Forbes would need to assert was compromised.
Data Authenticity Debate
Concerns range from the well-documented (delayed release, incomplete records) to the speculative (wholesale data fabrication). Below, ordered from most legitimate to most speculative.
Delayed and Incomplete Data Release
Inmarsat's satellite data wasn't published until May 27, 2014, over two months after the disappearance, and only after sustained pressure from passengers' families (led by Sarah Bajc) and independent researchers. The released 47-page document was incomplete: fields were missing, only a small number of pre-flight records were included, and comparable data from previous MH370 flights wasn't provided. It wasn't until June 2017, three years later, that what researchers believe is the complete, unredacted log became available, obtained by engineer Victor Iannello through unofficial channels.
Novel Methodology Concerns
The BFO Doppler analysis used to determine the southern corridor was developed specifically for MH370; it had never been used for aircraft tracking before. Scott Morgan (former US Air Force Rescue Coordination Center commander) noted: "There simply isn't enough information in the report to reconstruct the original data. We don't know what their assumptions are." The north/south determination relies on the Inmarsat-3 F1 satellite's slight orbital wobble (~1.6° inclination) creating an asymmetric Doppler signature, a small signal. Independent physicist Viktor Toth confirmed the approximate search area but stated he "was not able to find any rationale for preferring the southerly route over the northern route" from the data alone.
The Non-Uniqueness Problem
BFO values depend on both the aircraft's horizontal velocity and vertical speed (rate of climb or descent). Since vertical speed can be adjusted to fit against many different ground speeds or headings, multiple flight paths can produce the same BTO/BFO values. The data constrains possibilities but can't uniquely identify a flight path between hourly handshakes. The "best fit" depends on assumptions about aircraft behaviour (autopilot mode, speed, altitude) that are themselves uncertain. Even among researchers who accept the data as genuine, there is significant disagreement about what it means.
The Anomalous SDU Log-On Events
The SDU performed two unexpected log-on requests (at 18:25 UTC and 00:19 UTC), both without preceding log-off signals. That's the critical detail. Normal satellite handovers produce log-on/log-off pairs as the aircraft transitions between beams. MH370's previous flight (MH371) recorded 22 such routine handovers between departure and arrival. But the 18:25 and 00:19 events on MH370 weren't handovers. They were cold restarts: the SDU lost power, went dark, and rebooted. The absence of a log-off signal before each restart indicates sudden power interruption, not a routine beam switch. The official explanation attributes the 00:19 restart to the APU auto-starting after dual engine fuel exhaustion. The 18:25 restart has no official explanation at all. BFO transient behaviour during the 18:25 log-on suggests the SDU had cooled significantly before powering back on, consistent with either prolonged power loss or cabin depressurisation. That question remains open.
The Spoofing Vulnerability
Jeff Wise identified a technical vulnerability: the Boeing 777's SDU receives navigation data from the Inertial Reference System via an accessible cable in the electronics bay below the cockpit. By disconnecting this cable and substituting false position/velocity data, an attacker could alter the SDU's Doppler precompensation, making a northbound aircraft's BFO signature resemble a southbound one. Victor Iannello's calculations confirmed that modifying the SDU's satellite inclination parameter could make "a flight to Kazakhstan create BFO values similar to those actually logged." The French Gendarmerie Air Transport investigators took this seriously enough to seek approval to travel to the US "to check if a US company sells software capable of reprogramming or hacking the Satcom," the first government body known to investigate SATCOM spoofing. No public findings have been released. See our dedicated page on the French judicial investigation for the full scope of their inquiry. The counter-argument: BFO is measured at the Perth ground station, not stored on the aircraft, but this is precisely the signal that spoofed precompensation would alter.
The Implicit Criticism: 120,000 km² Searched, Nothing Found
Perhaps the most powerful challenge to the Inmarsat-derived search area is empirical: over 120,000 sq km of ocean floor has been searched along the 7th arc without finding the main wreckage. The ATSB's primary 60,000 km² search area incorporated 97% of the probability function derived from the satellite data. The Malaysian Safety Investigation Report ultimately concluded it was "unable to draw definitive conclusions about what happened to flight MH370." Some researchers attribute the failure to end-of-flight modelling errors (dive vs. long-range glide), while others see it as evidence that the underlying flight path interpretation contains fundamental errors.
Nine Issues with the Satellite Log
Independent researchers at mh370whathappened.com conducted a systematic audit of the ACARS satellite transaction log and identified nine distinct anomalies. Several of these have no benign explanation and haven't been addressed by any official investigation.
28 minutes of missing ACARS transactions. Between 18:15 and 18:43 UTC, no ACARS messages appear in the log. The system was active before and after this window. The gap coincides with the period when the aircraft's transponder was switched off and the plane deviated from its filed route. Whatever happened during those 28 minutes left no ACARS trace.
Post-flight data amendments. The ACARS log shows evidence of modifications made after the flight. Entries were amended in ways that don't match how the system records transactions in real time. If the raw data was altered after the fact, the question is who did it and why.
MH370's transactions weren't encrypted. MH371's were. The same aircraft (9M-MRO) flew as MH371 from Beijing to Kuala Lumpur earlier that day. That flight's ACARS log contained 644 rows, all encrypted. MH370's outbound leg? Unencrypted. Same plane, same ACARS system, same day. The encryption was present on one flight and absent on the next. Nobody has explained the discrepancy.
The re-powered SDU omitted the aircraft registration. When the SDU rebooted at 18:25 UTC after losing power, its log-on request didn't include the aircraft's registration number (9M-MRO). Under normal operation, the SDU identifies the aircraft during the handshake sequence. This omission is consistent with either a corrupted boot or a device that wasn't connected to the aircraft's systems in the expected way.
These aren't speculative inferences. They're observable properties of the data itself. The 28-minute gap, the encryption discrepancy, the post-flight amendments, and the missing registration are all verifiable by examining the ACARS log. The question isn't whether these anomalies exist. It's why the official investigation never addressed them.
Mixed: The delayed data release, novel methodology, and non-uniqueness problem are legitimate technical concerns raised by credible researchers. The spoofing vulnerability is technically documented but requires extraordinary assumptions about motive and capability. Wholesale fabrication claims (de Changy, Forbes) lack specific technical mechanisms and are contradicted by the Independent Group's validation of the data's internal consistency.
- Iannello, The Unredacted Inmarsat Satellite Data for MH370 (2017)
- Viktor Toth, Analysis of the MH370 Inmarsat Data
- Lyne, Final Two MH370 Communications Suggest Controlled Eastward Descent, Journal of Navigation (2024)
- Jeff Wise, The Vulnerability, Deep Dive MH370
- Ashton et al., The Search for MH370, Journal of Navigation (2014)
- mh370whathappened.com, 9 Issues with the Satellite Log
What This Means
Every underwater search zone, every drift model, every official reconstruction traces back to seven satellite handshakes. If those handshakes are read correctly, the wreckage is findable. If they aren't, $200M+ in search costs targeted the wrong ocean.
If the official interpretation is correct
The aircraft flew south for nearly 7 hours and crashed into the Indian Ocean near the 7th arc. The wreckage should be findable with sufficiently precise search parameters. The debris washing up in Africa is explainable by multi-year ocean drift. The 2026 search has a reasonable chance of success.
If the data was manipulated
The entire underwater search (240,000+ sq km, $200M+ spent) was based on false premises. The aircraft's actual fate would be unknown, and the search would be the largest misdirection in aviation history. This would imply state-level capability and motivation to alter satellite infrastructure data.
The 2026 search as a test: Ocean Infinity's current search is, in effect, a test of the Inmarsat data's validity. If wreckage is found in the predicted area, the data and its interpretation are vindicated. If the search fails for a third time, the questions about the data's reliability will intensify.
Related Topics
Where the wreckage washed ashore and what drift modelling reveals.
Live tracking of the 2026 deep-sea search informed by this data.
The reconstructed flight path and competing theories of what happened.
The judicial inquiry that took SATCOM spoofing seriously enough to investigate.